Process for making a voltage dependent resistor

ABSTRACT

A process for making a voltage dependent resistor which has a zinc oxide sintered body which itself has voltage dependent properties. The process is made up of the steps of: (1) providing a formed body of a powder mixture having as a major part zinc oxide and additive; (2) coating on the side surfaces of the formed body a paste having as the solid ingredient composition at least one member selected from the group of a) more than 50 mole % of silicon dioxide (SiO2), and less than 50 mole % of bismuth oxide (Bi2O3), b) the same composition as that of said additive, c) more than 30 mole percent of antimony oxide (Sb2O3) and less than 70 mole percent of bismuth oxide (Bi2O3), and d) more than 50 mole percent of indium oxide (In2O3) and less than 50 mole percent of bismuth oxide (Bi2O3); (3) sintering the coated body; and (4) applying electrodes to opposite surfaces of the sintered body.

Matsuoka et a1.

PROCESS FOR MAKING A VOLTAGE DEPENDENT RESISTOR Assignee:

Filed:

Appl. No.: 428,737

Inventors: Michio Matsuoka; Gen ltakura;

Atsushi Iga; Takeshi Masuyama, all of Osaka, Japan Matsushita Electric Industrial Co.,

Ltd., Osaka, Japan Dec. 27, 1973 Apr. 13, 1973 June 15, 1973 Foreign Application Priority Data Dec. 29, 1972 Dec. 29, 1972 Japan 47-483610 Japan t l 47483619 Japan..... 48-4842488 Japan 48-4868066 U.S. Cl 29/621, 29/610, 252/518,

Int. Cl ..H01C l/l4, H01c 17/00 Field of Search 29/610, 621; 338/20, 21, Y

References Cited UNITED STATES PATENTS Matsuoka et al. 29/621 X Matsuoka 338/20 Matsuoka et al. 252/518 1 Mar. 25, 1975 3,663,458 5/1972 Masuyama et a1 252/518 3,760,318 9/1973 Masuyama et al..... 29/610 X 3,764,566 10/1973 Matsuoka et a1. 252/518 3,778,743 12/1973 Matsuoka ct all 338/ Primary Examiner-Richard J. Herbst Assistant Examiner-Victor A. Di Palma Attorney, Agent, or Firm-Wenderoth, Lind & Ponack [5 7] ABSTRACT A process for making a voltage dependent resistor which has a zinc oxide sintered body which itself has voltage dependent properties. The process is made up of the steps of: (1) providing a formed body of a powder mixture having as a major part zinc oxide and additive; (2) coating on the side surfaces of the formed body a paste having as the solid ingredient composition at least one member selected from the group of a) more than 50 mole of-silicon dioxide (SiO and less than 50 mole of bismuth oxide (Bi O b) the same composition as that of said additive, c) more than mole percent of antimony oxide (Sb O and less than 70 mole percent of bismuth oxide (Bi- 0 and d) more than mole percent of indium oxide (1n O and less than 50 mole percent of bismuth oxide (Bi O (3) sintering the coated body; and (4) applying electrodes to opposite surfaces of the sintered body.

5 Claims, 1 Drawing Figure PATENIED MAR 2 5 I975 I l l PROCESS FOR MAKING A VOLTAGE DEPENDENT RESISTOR This invention relates to the preparation of a voltage dependent resistor the properties of which are due to the bulk thereof, and more particularly to a varistor comprising a zinc oxide sintered body having a high resistance layer of a composition such as silicon dioxide, antimony oxide or indium oxide on the side surface of the sintered body.

Various voltage dependent resistors such as silicon carbide varistors, selenium rectifiers and germanium or silicon p-n junction diodes have been widely used for stabilization of voltage or current of electrical circuits. The electrical characteristics of such a voltage dependent resistor are expressed by the relation:

l =(V/C)" where V is the voltage across the resistor, I is the current flowing through the resistor, C is a constant corresponding to the voltage at a given current and exponent n is a numerical value greater than 1. The value of n is calculated by the following equation:

1o( 2/ 1) g( 2/ 1) where V, and V are voltages at a given currents l and I respectively. The desired value of C depends upon the kind of application to which the resistor is to be put. it is ordinarily desirable that the value of n be as large as possible since this exponent determines the degree to which the resistors depart from ohmic characteristics.

There have been known voltage dependent resistors comprising sintered bodies of Zinc oxide with or without additives and having silver paint electrodes applied thereto, as disclosed in the U.SfPat. No. 3,496,512. The non-linearity of such voltage dependent resistors is attributed to the interface between the sintered body of zinc oxide with or without additives and the silver paint electrode and is controlled mainly by changing the composition of said sintered body and said silver paint electrode. Therefore, it is not easy to control the C value over a wide range after the sintered body is prepared. Similarly, in the voltage dependent resistors comprising germanium or silicon p-n junction diodes it is difficult to control the C-value over a wide range because the non-linearity of these voltage dependent resistors is not attributed to the bulk thereof but to the p-n junction. On the other hand, silicon carbide varistors have non-linearity due to the contacts among individual grains of silicon carbide bonded together by a ceramic binding material i.e. to the bulk and are controlled with respect to the C-value by changing the dimension in the direction in which the current flows through the varistors. The silicon carbide varistors, however, have a relatively low n-value ranging from 3 t0. 9am! are answe xfit painanqm wzimatme:

sphere, especiallyfor the purpose of obtaining a lower C-value. In US. Pat. Nos. 3,663,458, 3,669,058, 3,637,529, 3,632,528, 3,634,337 and 3,598,763, there have been disclosed voltage dependent resistors comprising sintered bodies of zinc oxide with additives such as bismuth oxide, uranium oxide, strontium oxide, lead oxide. barium oxide, cobalt oxide and manganese oxide. The non-linearity of such voltage dependent resistors is attributable to the bulk thereof and is independent of the interface between the sintered bodies and the electrodes. Therefore, it is easy to control the C- value over a wide range by changing the thickness of the sintered body itself. Such voltage dependent resistors of the bulk type have more excellent properties with respect to the n-value, transient power dissipation and AC power dissipation than do SiC varistors.

A disadvantage of the zinc oxide voltage-dependent resistors is their poor stability in an electric load life test in a high ambient humidity. When DC. power is applied to the Zinc oxide sintered body in a high ambient humidity, the sintered body shows a decrease in the surface electrical resistance. This decrease causes in particular an increase in the leakage current in the zinc oxide voltage-dependent resistor of the bulk type and results in a poor non-linear property. The deterioration of the non-linear property of the voltage-dependent resistor occurs even at a load of low power such as a load lower than 0.01 watt in a high ambient humidity, for example 90 percent R.H at C. Therefore, it is necessary that the sintered body is completely protected against outside moisture by a protective coating.

Another disadvantage of the zinc oxide voltage dependent resistors aforesaid exists in their poor ability to withstand impulse current. When an impulse, wave is applied to the zinc oxide sintered body, the sintered body suffers a flashover along its side surface at an impulse voltage above SOOV/mm, and despite no deterioration in the interior of sintered body the side surface of the sintered body is heavily damaged. The poor ability to withstand impulse current is unfavorable particu larly for application of the varistor as a lightning arrester,

There is other prior art that relates to a voltage dependent resistor comprising a sintered body comprising zinc oxide and other additives and being characterized by a high C-value, high n-value, high stability with respect to temperature, humidity and electric load, and good ability to withstand impulse current. Such a resistor is disclosed in U.S. Pat. No. 3,760,318. More specifically, a zinc oxide sintered body according to said U.S. Pat. No. 3,760,318 has Li ions or Na ions diffused into said sintered body from the side surface thereof at a temperature of 600C to l0O0C. This diffusing process inevitably results in lowering the n-value of the resultant resistor in the current region lower than 10 uA. The low n-value in such low current region is undesirable for an application requiring low leakage current.

An object of the present invention is to provide a method for making a voltage dependent resistor characterized by a high stability with respect to a dc. load in high humidity and a good ability to withstand impulse current.

Another object of the present invention is to provide a method for making a voltage dependent resistor characterized by a high n-value even in a low current region and a high stability with respect to a dc. load in high humidity and a good ability to withstand impulse current.

These and other objects of the invention will become apparent upon consideration of the following description taken together with the accompanying drawing in which the single FIGURE is a partly cross-sectional view of a voltage-dependent resistor in accordance with the invention.

Before proceeding with a detailed description of the manufacturing process for the voltage-dependent resistor contemplated by the invention, the construction of the resultant resistor will be described with reference to the aforesaid FIGURE wherein reference character 10 designates, as a whole, a voltage-dependent resistor comprising, as its active element, a sintered body having surfaces consisting of a side surface 2 and opposite end surfaces 3 and 4 to which a pair of electrodes 5 and 6 are applied. Said sintered body 1 is prepared in a manner hereinafter set forth and has a high resistance layer 11 at said side surface 2 and can have any crosssectional form such as circular, square or rectangular.

The process for making a voltage dependent resistor of a bulk type characterized by a high humidity resistance and a good ability to withstand current surges according to the invention comprises: (1 providing a formed body of a powder mixture comprising, as a major part, zinc oxide, and an additive including BiO (2) coating on the side surfaces of said body a paste comprising, solid ingredient composition, at least one member selected from the group consisting of a more than 50 mole percent of silicon dioxide (SiO and less than 50 mole percent of bismuth oxide (Bi O b) the same composition as that of said addition, c) more than 30 mole percent of antimony oxide (Sb O and less than 70 mole percent of bismuth oxide (Bi O and d) more than 50 mole percent of indium oxide (In O and less than 50 mole percent of bismuth oxide (Bi O (3) sintering said coated body; and (4) applying two electrodes to the opposite end surfaces of said sintered body. I

Said zinc oxide sintered bodywhich itself has voltage dependent properties can be prepared by using a composition described in U.S. Pat. Nos. 3,663,458, 3,669,058, 3,636,529, 3,632,528, 3,634,337 and 3,598,763. Among various compositions, a better result can be obtained with a composition consisting essentially of, as a major part, 80.0 to 99.9 mole percent of zinc oxide and, as an additive, 0.05 to 10.0 mole percent of bismuth oxide (Bi O and 0.05 to 10.0 mole percent, in total, of at least one member selected from the group consisting of cobalt oxide (C), manganese oxide (MnO), antimony oxide (Sb O barium oxide (BaO), strontium oxide (SrO) and lead oxide (PbO).

According to the present invention, the resultant resistor has an excellent ability to withstand current surges in an impulse current test, when said coating paste comprises, as the solid composition, 70 to 95 mole percent of silicon dioxide (SiO and 30 to mole percent of bismuth oxide (Bi O Similarly, the ability to withstand surge current can be improved greatly by using coating paste comprising, as the solid ingredient composition, 70 to 95 mole percent of antimony oxide (Sb O and 30 to 5 mole percent of bismuth oxide 2 3)- According to the present invention, the ability to withstand surge current can be further improved by using coating paste comprising, as the solid ingredient composition, 50 to 95 mole percent of silicon dioxide (SiO 2 to 45 mole percent of antimony oxide (Sb O and 2 to mole percent of bismuth oxide (Bi O It has been discovered according to the invention that the DC. stability in high humidity and the ability to withstand surge current of the resultant resistor is improved when said powder mixture consists essentially of, asa major part, 99.9 to 80.0 mole percent of zinc oxide (ZnO) and, as an additive, 0.05 to 10.0 mole percent of bismuth oxide (Bi O and 0.05 to [0.0 mole percent, in total, of at least one member selected from the group consisting of cobalt oxide (C00), manganese oxide (MnO) antimony oxide (Sb- 0 barium oxide (BaO), strontium oxide (SrO) and lead oxide (PbO).

The sintered body 1 can be prepared by a per se well known ceramic technique. The starting materials comprising zinc oxide powder and additives such as bismuth oxide, cobalt oxide, manganese oxide, antimony oxide, barium oxide, strontium oxide, lead oxide, uranium oxide and tin oxide are mixed in a wet mill so as to produce a homogeneous mixture. The mixtures are dried and pressed in a mold into desired shapes at a pressure from 100 kg/cm' to 1000 kg/cm When a rodshaped resistor is desired, the mixed slurry can be fabricated into the desired shape by extruding and then dried. The mixtures may be preliminarily calcined at a temprature of 700 to 1000C and pulverized for easy fabrication in the subsequent pressing step. The mixtures may be admixed with a suitable binder such as water, polyvinyl alcohol, etc.

After the mixtures are formed into the desired shapes, the formed bodies are coated, on the side surfaces, with a paste including powder having the same composition as said additive, or a combination of bismuth oxide with silicon dioxide, antimony oxide or indium oxide, so as to form a high resistance layer at the side surfaces after sintering. Said paste comprises, as the solid ingredient composition, at least one member selected from the group consisting of a more than 50 mole percent of silicon dioxide (SiO and less than 50 mole percent of bismuth oxide (Bi O b) the same composition as that of said additive, c) more than 30 mole percent of antimony oxide (Sb O and less than mole percent of bismuth oxide (Bi O and d) more than 50 mole percent of indium oxide (In O and less than 50 mole percent of bismuth oxide (Bi O and, as a binding material, an organic resin such as epoxy, vinyl or phenol resin in an organic solvent such as butyl acetate, toluene or the like. Said silicon dioxide, bismuth oxide, antimony oxide and indium oxide can be replaced, respectively, with any silicon compound, bismuth compound, antimony compound and indium compound such as an oxalate, carbonate, nitrate, sulfate, iodide, fluoride or hydroxide which is converted into the corresponding oxide at the sintering temperature. I

After being coated with said paste, the formed bodies are sintered in air at a temperature of 1000 to l450C for 1 to 5 hours, and then furnace-cooled to room temperature. The sintering temperature is determined based on the desired electrical resistivity, nonlinearity stability and the thickness of the high resistance layer formed at the side surface of the sintered body. Also, the electrical resistivity can be reduced by airquenching from the sintering temperature to room temperature. The sintered body has non-ohmic resistance due to the bulkitself. Therefore, its C-value can be changed without impairing the n-value by changing the distance between said opposite end surfaces. A shorter distance results in a lower C-value. The coating paste forms a high resistance layer, as can be proved by measurement of the resistance distribution in the crosssection of the sintered body, which will show a high resistance at the side surface of the sintered body. The high resistance layer is controlled so as to have a thickness more than 10p. Particularly, it can be shown from an x-ray analysis of the cross-sectional portion of sintered body, that the paste comprising a combination of silicon dioxide and bismuth oxide, or antimony oxide and bismuth oxide forms a layer having a thickness of more than 3 1. and that said layer comprises, in a region electrolytic plating of Ag, Cu, Ni, Sn etc. vacuum evaporating of Al, Zn, Sn etc. and flame spraying of Cu, Sn, Al, Zn etc. in accordance with the prior well known techniques.

Lead wires can be attached to the electrodes in a per se conventional manner by using conventional solder. It is convenient to employ a conductive adhesive comprising silver powder and resin in an organic solvent in order to connect the lead wires to the silver electrodes. The n-value of a voltage-dependent resistor according to this invention does not deteriorate even in a low current region due to the introduction of the covering layer at the side surface of the sintered body, and it has a high stability with respect to temperature and humidity and in the load life test, which is carried out at 70C, 90 percent RH at a rating power for 500 hours. The nvalue and C-value do not change appreciably after the load life test. From a surge test, which is carried out by applying a 4X 10 p.sec impulse current twice, it is shown that this voltage-dependent resistor has the ability to withstand surges of more than 2000A/cm EXAMPLE 1.

Starting materials listed in Table 1 were mixed in a wet mill for 5 hours. Each mixture was dried and pressed in a mold into a disc of mm in diameter and 25 mm in thickness at a pressure of 340 kg/cm The pressed bodies had the side surface covered by coating paste including solid ingredients listed in Table 1. and

were dried. Then, the bodies were sintered in air for 5 hours at 1200C and furnace-cooled. The sintered bodies were lapped to the thickness listed in table 1 by lapping the opposite end surfaces thereof with silicon carbide abrasive having a particle size of 600 mesh. The opposite end surfaces of the sintered discs were provided with a spray metallized film of aluminum by a per se well known technique. The electric characteristics of the resultant resistors are, shown in Table 1. It will be readily understood that the C-value changes in proportion to the thickness of the sintered body.

Size of disc: 32 mm in dia.

Thickness of high resistive layer: 30a

Table 1 Composition Solid Ingredient Thickness C(V) n of Sintered of Paste of Sintered ody Body (mol. (mol. (mm) (at lmA) (0.1-lmA) 810 5 15 10 302 14 Bi O (50) 20 605 15 SiO (90) 5 153 l5 l0 BIO 16 B1 0 10) 20 605 16 SiO (I00) 5 14 ZnO (99.0) 10 310 15 B1 0; 0.5) B1 0 O) 20 615 15 C00 0.5) Sb n (90) 5 150 15 10 300 15 Bi O (I0) 20 603 15 T1303 (90) 5 145 14 10 300 14 B1 0, 10) 20 600 15 '0 (72) 5 16 Sb O (20) 10 315 16 Bi O 8) 20 615 16 S10; (90) 5 510 44 10 1025 45 Bi O,-, 10) 20 2040 45 2110 (97.5) Sb O (90) 5 500 45 Bi on 015) 10 lOlO 45 C00 0.5) Bi O (10) 20 2010 46 MnO 0.5) ["203 (90) 5 505 45 Sb O 1.0) 10 1010 44 Bi O (10) 20 2015 46 2 (72) 5 515 46 Sb O (20) 10 1025 46 81 0 8) 20 2040 46 ZnO (99.0) Sb O (90) 5 250 22 B1 0 0.5) 10 505 22 ,MnO (0.5) Bi- O (10) 20 1000 23 ZnO (99.0) S10 (90) 5 240 8.2 13i O (0.5) 10 490 8.4 sb lo (0.5 Bi O 10 20 985 8,4 ZnO (99.0) "203 (90) 5 200 10 Bi O 0.5) 10 410 10 BaO 0.5) B50 (10) 20 815 10 ZnO (99.0) SiO (72) 5 205 11 Bi O 0.5) Sb O (20) 10 400 1 1 8) 20 810 12 SrO 0.5)

Bi Q

EXAMPLEZ EXAMPLE 3 Starting materials of Table 3 were fabricated into voltage dependent resistors by the same process as that of Example 1. Then the tests were carried out by the same methods as those of Example 2. The electric characteristics of the resultant resistors are shown in Table 3.

Size of disc: 32 mm in dia. and 20 mm in thickness Sintering: 1200C for hours Thickness of high resistance layer: 30p.

TABLE 2 Composition Electric Characteristics of Resultant of Sintered Solid Ingredient Resistor Body of Paste C(V) n Impulse Boiling (mol. 70) (mol (at lmA) 0.11mA Withstand, Test (KA) AC SiO (50) Bi o (50) 605 15 20 5.0 SiO (60) Bi O (40) 605 15 20 4.7 SiO (70) B g fl (30) 600 15 25 4.7 SiO (80) Bi n (20) 600 16 30 3.8 SiO (90) Bi O 605 16 35 2.9 SiO (95) Bi O 5) 610 16 30 3.2 Si0 (100) Bi o 0) 615 30 3.5 Sb O (30) Bi O (70) 600 14 -53 ZnO (99.0) S11 0 (50) Bi O (50) 600 14 4.5 Bi 0,,(05) Sb 0 (70) Bi O 600 15 25 3.5 C00 (0.5) Sb O (90) Bi O (10) 603 15 2.7 Sb O (95) Bi O (5) 605 15 30 3.0 Sb O (100) Bi 03' (0) 610 14 25 3.3 ln O Bi O (50) 595 14 '20 5.7 ln O (70) Bi O (30) 600 14 25 4.3 ln O (90) BL O (10) 600 15 35 3.1 ln O (95) B1 0 (5) .600 15 30 3.4 [n 3 (100) Bi O (0) 610 14 30 '3.5 SiO (50) B90 (50) 1960 42 25 5.5 SiO Bi O (40) 1980 42 30 4.8 SiO Bi O (30) 2000 44 35 3.9 SiO Bi O (20) 2100 44 40 3.2 SiO Fi Q-i 10) 2040 45 40 1.5 Si0 Bi O, 5) 2040 45 35 2.1 SnO (97.5) SiO Bi O 0) 2030 44 30 2.3 B1 04 0.5) Sb O (30) Bi O (70) 1980 44 25 51 C00 0.5) Sb O (50) Bi O (50) 2000 44 30 4.9 MnO (0.5) Sta-10: (70) Bi O (30) 2000 45 35 3.8 Sh- OM 1.0) S11 0, (90) Bi O (10) 2010 46 40 25 Sb O (95) B50 5) 2015' 45 40 3.l Sb- O (100) Bi On 0) 2020 45 30 3.5 ln 0 (50) Bi O (50) 1990 44 25 5.3 1:1 0 (70) Bi O (30) 2005 44 30 4.9 ln O (90) Bi O- (10) 2015 46 40 3.1 In O (95) B1 0 5) 2015 45 40 3.4 ln Og (100) Bi O 0) 2000 45 25 3.4

Si0 Sb O Bi O 50 45 5 600 15 30 -4.4 ZnO (99.0) 50 30 20 600 15 30 4.8 Bi O (0.5) 95 3 2 615 16 35 3.2 I CoO (0.5) 95 2 3 615 16 40 3.4

58 40 2 610 15 3.5 3.0 78 2 20 610 15 40 2.5 72 20 8 620 17 45 1.7 50 45 5 2050 44 40 3.4 ZnO (97.5) 50 30 20 2065 45 45 2.8 Bi 0. (0.5) 95 3 2 2045 45 50 2.7 C00 (0.5) 95 Y 2 3 2075 46 50 2.7 MnO (0.5) 58 40 2 2060 44 50 2.0 Sb ,O 1.0) 78 2 20 2080 46 55 1.2 72 20 8 2100 48 60 .5

The fabrication process and testing method were the same as those of Example 2 and the thickness of the high resistance layer was varied with the results as shown in Table 4. It is easily understood that the ability to withstand impulses increases with an increase in the thickness of the high resistance layer and the rate of change of the C-value caused by the boiling test decreases with an increase of thickness of the high resis- ,1 EXAMPLE 5 Starting materials of Table 5 were fabricated into voltage dependent resistors by the same process as in Example 1. The pressed bodies were sintered at a temperature between 1000C to 1450C for 5 hours after covering the side surface with coating pastes as listed in Table 5. The test conditions were the same as those of Example 2. The electric characteristics of resulting tame y resistors are shown in Table 5.

Size of dlscz 32 mm m dia. and 20 mm in thickness si of di 32 mm i di d 20 S ntermg: 1200C for hours Thickness of high resistive layer; 3011 TABLE 4 Composition Solid Ingredient Thickness Electric Characteristics of of Sintered of Paste of High- Resultant Resistor Body (mol Resistive C(V) n Impulse Boiling (mol layer (at 1 mA) 0.1-1 mA Withstand Test (M) (KA) AC 600 16 30 4.3 Si0 (90) 30 605 16 35 2.9 B1 0; (10) 100 605 16 40 3.2 300 615 16 50 l.2 10 600 14 30 4.2 Sb O (90) 30 603 35 2.7 ZnO (99.0) 31.0., (10) 100 605 15 40 2.2 Bi O 0.5) 300 610 I5 45 l.7 CoO (0.5 10 590 15 4.8 [T1203 1 (90) 600 15 3.1' B1 0. (10) 100 605 15 40 3.3 300 610 16 2.7 10 605 17 35 3.3 Si0 (72) 30 620 17 45 l.7 Sb O 100 620 17 1.2 Bi O 8) 300 630 18 60 1.0 10 200 43 30 2.1 SiO 90) 30 2040 45 40 l .5 Bi- O (10) 100 2070 45 45 -1 .1 300 2100 46 45 .5 10 1950 43 30 3.3 ZnO (97.5) Sbgon (90) 30 2010 46 '40 -25 Bi 0 (0.5 B1203 (I0) 100 2030 46 40 2.0 (00 (0.5) 300 2050 46 50 l.6 MnO (0.5) 10 2000 44 30 4.7 sh oflt 1.0) 111 0;, (90) so 2015 46 40 -3.1 Bipu (I0) 100 2050 46 12 300 2100 47 l.8 10 2050 46 50 '12 SiO (72) 30 2100 48 60 0.5 Sb O (20) 100 2120 50 0.5 Bi o (8) 300 2150 50 0.4

TABLE 5 Composition Solid Ingredient Sintering Electric Characteristics of of Sintered Resultant Resistor Body of Paste Temp. C (V) n Im ulse Boiling (mo1.%) (mol. (C) (at 1 mA) 0.1-lmA Wit stand Test AC(%) 1000 1200 11 15 -9.5 1100 850 l4 17 7.2 SiO, 50) 1200 605 15 20 5.0 Bi o (50) 1300 420 13 18 5.1 1450 280 1 l 18 5.3 1000 1220 13 20 7.7 1100 870 14 25 4.1 SiO 1200 605 I6 35 -29 Bad, 10) 1300 450 16 35 -2.9 1450 300 15 30 3.5 1000 1250 12 20 7.0 l 900 14 25 -5.l Si0 (100) 1200 615 I5 30 3.5 Bi,0 0) 1300 470 I4 23 3.7 1450 330 14 20 4.0 1000 1200 ll 15 8.1

Table 6 Contlnued Composition of Sintcred body Solid Ingredient Sinten'ng Electric Characteristics of (mol. 70) of Paste Temp. Resultant Resistor lm ulse Boiling Further C (V) n Wit stand Test ZnO Bi O Additives (moi. (C) (at lmA) 0.1-lmA (KA) AC (70) 97.0 0.5 MnO 0.5 MnO 1200 2600 50 60 0.5

Sb O 1.0 Sb O 40 SnO 0.5 Sn0 30 Bi O 10 CoO 0.5 C00 10 97.0 0.5 MnO 0.5 MnO 10 1200 2800 50 60 0.5

Sb O 1.0 Sb,0;, 60 0,0,, 0.5 Cr O l0 Bi Q 10 C0D 0.5 CoO 5 96.5 0.5 MnO 0.5 MnO 5 1200 4400 55 70 0.3

smo. 1.0 Sb O 25 C110 0.5 r 0 5 SiO; 0.5 0 50 B1 0. 5 C0D 0.5 CoO 5 MnO 0.5 MnO 5 94.0 0.5 Sb O 1.0 Sb O 1200 5600 60 70 -03 CF10 0.5 Cr O 3 SiO 2.0 SiO 60 M0 1.0 NiO 2 Bi O 1000 3800 35 35 l .2 C00 0.5 C00 25 98.0 0.5 MnO 0.5 MnO 25 1200 1800 40 50 0.8 0.5 Sb,O 41 1.3

What we claim is:

l. A process for making a voltage dependent resistor comprised of a zinc oxide sintered body which itself has voltage dependent properties, said process comprising: (1) providing a formed body of powder mixture comprising, as a major part, zinc oxide and the remainder being an additive; (2) coating on the side surface of said body a paste having a solid ingredient composition of at least one member selected from the group consisting of a) more than 50 mole percent of silicon dioxide (SiO and less than 50 mole percent of bismuth oxide (Bi O b) the same composition as that-of said additive, c) more than mole percent of antimony oxide (Sb O- and less than 70 mole percent of bismuth oxide (Bi O- and d) more than 50 mole percent of indium oxide (ln O and less than 50 mole percent of bismuth oxide (Bi O (3) sintering said coated body; and (4) applying two electrodes to the opposite end surfaces of said sintered body.

2. A process according to claim 1, in which the coating paste has a solid in gredient composition of 70 to 95 mole percent ofsilicon dioxide (SiO and 30 to 5 mole percent of bismuth oxide (Bi O 3. A process according to claim 1, in which the coating paste has a solid ingredient composition of to mole percent of antimony oxide (Sb O and 30 to 5 mole percent of bismuth oxide (Bi O 4. A process according to claim 1, in which the coating paste has a solid ingredient composition of 50 to 95 mole percent of silicon dioxide (SiO 2 to 45 mole percent of antimony oxide (Sb O and 2 to 20 mole percent of bismuth oxide (B50 5. A process according'to claim 1 which said powder mixture consists essentially of, as a major part, 99.9 to 80.0 mole percent of zinc oxide (ZnO) and, as an additive, 0.05 to 10.0 mole percent of bismuth oxide (Bi O and 0.05 to 10.0 mole percent of at least one member selected from the group consisting of cobalt oxide (C00), manganese oxide (MnO) antimony oxide (Sb O barium oxide (BaO), strontium oxide (SrO) and lead oxide (PbO). 

1. A PROCESS FOR MAKING A VOLTAGE DEPENDENT RESISTOR COMPRISED OF A ZINC OXIDE SINTERED BODY WHICH ITSELF HAS VOLTAGE DEPENDENT PROPERTIES, SAID PROCESS COMPRISING: (1) PROVIDING A FORMED BODY OF POWDER MIXTURE COMPRISING, AS A MAJOR PART, ZINC OXIDE AND THE REMAINDER BEING AN ADDITIVE, (2) COATING ON THE SIDE SURFACE OF SAID BODY A PASTE HAVING A SOLID INGREDIENT COMPOSITION OF AT LEAST ONE MEMBER SELECTED FROM THE GROUP CONSISTING OF A) MORE THAN 50 MOLE PERCENT OF SILICON DIOXIDE (SIO2) AND LESS THAN 50 MOLE PERCENT OF BISMUTH OXIDE (BI2O3), THE SAME COMPOSITION AS THAT OF SAID ADDITIVE, C) MORE THAN 30 MOLE PERCENT OF ANTIMONY OXIDE (SB2O3) AND LESS THAN 70 MOLE PERCENT OF BISMUTH OXIDE (BI2O3), AND D) MORE THAN 50 MOLE PERCENT OF INDIUM OXIDE (IN2O3) AND LESS THAN 50 MOLE PERCENT OF BISMUTH OXIDE (BI2O3), (3) SINTERING SAID COATED BODY, AND (4) APPLYING TWO ELECTRODES TO THE OPPOSITE END SURFCES OF SAID SINTERED BODY.
 2. A process according to claim 1, in which the coating paste has a solid ingredient composition of 70 to 95 mole percent of silicon dioxide (SiO2) and 30 to 5 mole percent of bismuth oxide (Bi2O3).
 3. A process according to claim 1, in which the coating paste has a solid ingredient composition of 70 to 95 mole percent of antimony oxide (Sb2O3) and 30 to 5 mole percent of bismuth oxide (Bi2O3).
 4. A process according to claim 1, in which the coating paste has a solid ingredient composition of 50 to 95 mole percent of silicon dioxide (SiO2), 2 to 45 mole percent of antimony oxide (Sb2O3) and 2 to 20 mole percent of bismuth oxide (Bi2O3).
 5. A process according to claim 1 which said powder mixture consists essentially of, as a major part, 99.9 to 80.0 mole percent of zinc oxide (ZnO) and, as an additive, 0.05 to 10.0 mole percent of bismuth oxide (Bi2O3) and 0.05 to 10.0 mole percent of at least one member selected from the group consisting of cobalt oxide (CoO), manganese oxide (MnO) antimony oxide (Sb2O3), barium oxide (BaO), strontium oxide (SrO) and lead oxide (PbO). 